The amplifier for AC three-phase motors includes a pulse-width modulation circuit for voltage, current, and frequency control. Figure 11-81 shows an example of this type of amplifier. From the diagram you can see that this circuit is designed specifically for a three-phase trapezoidal motor. The transistors in the amplifier are connected in an H-bridge configuration. The motor windings are connected as a three-phase wye with no external wires connected to the wye point. This type of motor is also called a star connection when it is used with brushless AC servomotors. 

The drive logic and PWM switching controller is shown in the diagram as a block that is identified as a logic and PWM circuit. This block shows six arrows pointing away from it and pointing to the transistors. These arrows represent the six circuits for the base of each of the six transistors. The block below the PWM circuit represents the current-sensing part of the amplifier. This section of the amplifier uses a recirculating chopper system to control the current in a manner that is similar to the chopper circuit in the DC amplifier. The signals for this section of the amplifier come from the voltage that is developed across the series resistors connected between the transistor section and the motors. As you know, the amount of current flowing to the motor will determine the amount of voltage drop across these resistors. 

This amplifier has a velocity amplifier that receives the original command signal for the amplifier and the velocity feedback. The op amp provides an output that represents the difference (error) between the command signal and the feedback signal. The output of the velocity amp is sent to the torque amp, where it is combined with the feedback from the current-sensing block. The output from this op amp is sent to the logic and PWM circuit block where it acts as the command signal. The position encoder provides the feedback signal for this block. This means that the velocity and position amplifiers are actually a closed-loop system within a closed-loop system. The gain for each of these amplifiers must be tuned so that the system has the best torque response and smooth acceleration and deceleration.

FIGURE 11-81 An AC servo drive amplifier specifically designed to operate with an AC trapezoidal brushless servomotor.

The feedback mechanism is generally a brushless DC tach generator, or an AC generator. Each of these feedback mechanisms provides smooth feedback voltages. If an encoder is used, its binary (digital) signal must be converted to an analog signal through a D/A converter or a frequency-to-analog F/A type converter if the signal is produced as a frequency.(end)